Shallow melt apparatus for semicontinuous czochralski crystal growth

ABSTRACT

In a single crystal pulling apparatus for providing a Czochralski crystal growth process, the improvement of a shallow melt crucible ( 20 ) to eliminate the necessity supplying a large quantity of feed stock materials that had to be preloaded in a deep crucible to grow a large ingot, comprising a gas tight container a crucible with a deepened periphery ( 25 ) to prevent snapping of a shallow melt and reduce turbulent melt convection; source supply means for adding source material to the semiconductor melt; a double barrier ( 23 ) to minimize heat transfer between the deepened periphery ( 25 ) and the shallow melt in the growth compartment; offset holes ( 24 ) in the double barrier ( 23 ) to increase melt travel length between the deepened periphery ( 25 ) and the shallow growth compartment; and the interface heater/heat sink ( 22 ) to control the interface shape and crystal growth rate.

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government has rights in this inventionpursuant to Contract No. DE-AC36-99GO-10337 between the United StatesDepartment of Energy and the Midwest Research Institute.

TECHNICAL FIELD

[0002] The present invention relates to growing crystals withoutencountering the problems normally associated with conventionalCzochralski crystal growth of: strong melt convection that causesdeteriorated crystal quality, constantly changing thermal conditions;segregation causing non-uniformity in dopant concentrations; a slowgrowth rate and an uncontrollable interface shape; slow turn aroundbetween crystal ingots; and significant loss of feedstock materials if agrowth run fails. The invention eliminates the foregoing problemsthrough the use of a shallow melt with a heat shield and an interfaceheater-heat sink to overcome microdefects, stress management and defectminimization, swift turnaround between crystal ingots, and the saving offeedstock materials through the use of a double barrier that offsetsconnecting holes for replenishing the melt.

BACKGROUND ART

[0003] Affecting crystal growth using the conventional Czochralski (CZ)process has been accomplished utilizing a pulling apparatus employingthe CZ technique comprising: A gas type chamber; a crucible for storinga semiconductor melt that is positioned inside the chamber; a heater forheating the semiconductor melt; and a pulling mechanism for pulling asingle crystal of the semiconductor. With this apparatus, a seed crystalof the single crystal of the semiconductor is immersed in thesemiconductor melt inside the crucible, whereupon the seed crystal isgradually pulled upwards, thereby growing a large diameter singlecrystal of the semiconductor having the same orientation as the seedcrystal.

[0004] As the Czochralski method evolved as the prevalent one for theoperation of pulling a silicon single crystal, a method of the versionadopted a double-wall crucible formed by disposing a cylindricalpartition wall in an outer crucible to operate the crucible by supplyingsolid or fused silicon as a raw material, either batch wise orcontinuously, into the crucible through a gap between the inner surfaceof the outer crucible and the outer surface of the cylindrical partitionwall and pulling a silicon single crystal from the molten mass ofsilicon in the cylindrical partition wall.

[0005] U.S. Pat. No. 5,871,581 discloses a single crystal pullingapparatus comprising:

[0006] a gas tight container;

[0007] a double crucible for storing a semiconductor melt inside the gastight container, the double crucible including an outer crucible and aninner crucible connected at a lower edge;

[0008] source material supply means, disposed between the outer crucibleand the inner crucible, for continuously adding source material to thesemiconductor melt; and

[0009] a flow restriction member, disposed inside the semiconductor meltregion between the outer crucible and the inner crucible, forrestricting the flow of the semiconductor melt.

[0010] A crucible for pulling silicon single crystal is disclosed inU.S. Pat. No. 5,720,809. The crucible is constructed by coaxiallydisposing a cylindrical partition wall inside an outer crucible forholding a molten mass of silicon as a raw material and operated byheating the outer crucible and meanwhile supplying the raw materialsilicon to the gap between the outer crucible and the cylindricalpartition wall and introducing the consequently produced molten mass ofsilicon into the interior of the cylindrical partition wall via apassage below the surface of the molten mass interconnecting the outercrucible and the inside of the cylindrical partition wall and meanwhilepulling the single crystal bar from the molten mass of silicon insidethe cylindrical partition wall, which double-wall crucible ischaracterized in that at least the cylindrical partition wall is formedof quartz glass having a hydroxyl group (OH group) content of not morethan 30 ppm.

[0011] There is a need in the art of crystal growth utilizing theCzochralski method to avoid the problems of: (1) strong melt convectionleading to deteriorated crystal quality (high oxygen content when asilica crucible is commonly used, and increased microdefects); 2)constantly changing thermal conditions even with synchronized cruciblelift; (3) segregation caused non-uniformity in dopant concentrations;(4) a slow growth rate (productivity) and a generally incontrollableinterface shape (for stress management and defect minimization); (5)slow turn around between crystal-ingots; and (6) a significant loss offeedstock materials if a growth run fails.

DISCLOSURE OF THE INVENTION

[0012] One object of the present invention is to provide asemicontinuous Czochralski crystal growth from a shallow melt apparatusthat overcomes the increase of microdefects during crystal growth.

[0013] Another object of the present invention is to provide asemicontinuous Czochralski method of crystal growth from a shallow meltapparatus that lessens stress management and minimizes defects in thecrystal growth.

[0014] A further object of the present invention is to provide asemicontinuous Czochralski method of crystal growth from a shallow meltapparatus that hastens turn around between crystal ingots.

[0015] A yet further object of the present invention is to provide asemicontinuous Czochralski method of crystal growth from a shallow meltapparatus that saves feed stock materials.

[0016] Another object yet still of the present invention is to provide asemicontinuous Czochralski method of crystal growth from a shallow meltapparatus that offsets connecting holes for replenishing the melt.

[0017] In general, the CZ crystal growth methods of the invention areaccomplished by the use of:

[0018] a crucible with a deepened periphery to prevent snapping (i.e., asudden uncontrollable motion) of a shallow melt to reduce turbulent meltconvection;

[0019] a double barrier to isolate heat in the feeding compartment fromthe growth compartment to melt solid feeding materials and minimizethermal impact to the growing crystal;

[0020] an interface thermal gradient control, wherein a shallow melt inthe growth compartment allows for a much better thermal gradient controlnear the interface; and

[0021] an interface heater or a heat sink under the growth compartment,to obtain a concave or a convex interface shape.

BRIEF DESCRIPTION OF DRAWINGS

[0022]FIG. 1 shows the apparatus (including crucible) for a conventionalCZ crystal growth process.

[0023]FIG. 2 depicts apparatus of the invention for providingsemicontinuous CZ crystal growth from a shallow melt.

[0024]FIG. 3 shows the melt motion where the interface energy is largerthan the surface energy (i.e., non-wetting crucible, θ>90 degrees),where the melt will tend to ball-up.

[0025]FIG. 4 depicts the asymmetric melt that ensues when both meltsurface energy and the interface energy are very high compared togravity potential energy.

[0026]FIG. 5 depicts a melt stabilized from a round or rectangularcrucible when a deepened periphery traps the melt around the edge andprevents snapping.

[0027]FIG. 6 depicts a top view of the growth department in a rounddouble barrier crucible for semiconductor ingot growth where meltconnecting holes are utilized outside of the growth compartment butinside of the feed compartment.

[0028]FIG. 7 depicts a side view of a rectangular double-barriercrucible for a sheet or ribbon-shaped crystal growth and shows the meltconnecting holes outside of the growth compartment but inside of thefeed compartment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] In the semicontinuous Czochralski crystal growth from a shallowmelt of the invention, the essential elements of the crucible comprisesa crucible with a deepened periphery to prevent snapping of a shallowmelt, and has a double barrier to isolate heat in the feedingcompartment from that in the growth compartment, and further contains aninterface thermal gradient control.

[0030] The invention will be better understood and the objects andfeatures thereof apparent by reference to the drawing figures, comparedto a conventional CZ crystal growth process utilizing a differentcrucible construction.

[0031] In this connection, reference is now made to FIG. 1 which shows aconventional CZ crystal growth apparatus and a melt crucible 10, as usedtherein. In this conventional process, a large quantity of feed stockmaterials must be preloaded in the deep crucible or melt crucible togrow a large ingot. This unfortunately causes problems such as, strongmelt convection leading to deteriorated crystal quality (high oxygencontent when a silica crucible is used and increased microdefects),constantly changing thermal conditions even with synchronized cruciblelift, segregation caused by non-uniformity in the dopant concentrations;a slow growth rate (productivity) and a generally incontrollableinterface shape (for stress management and defect minimization), slowturn around between crystal ingots and a significant loss of feed stockmaterials if a growth run fails.

[0032] On the other hand, and by contrast, semicontinuous CZ crystalgrowth from a shallow melt of the invention, as depicted in theapparatus of FIG. 2 presents solutions to each of the foregoingproblems. As can be seen from FIG. 2, the invention apparatus features ashallow melt crucible 20 (particularly in the region immediately underthe crystal/liquid interface) with an insulation or heat shield 21 andan interface heater/heat sink 22, and these features overcome theproblems of: strong melt convection leading to deteriorated crystalquality; a slow growth rate and a generally incontrollable interfaceshape; slow turnaround between crystal ingots and a significant loss offeed stock materials if a growth run fails. The double barrier 23 withoff setting connecting holes 24 for replenishing the melt eliminates theproblem of constantly changing thermal conditions even with synchronizedcrucible lift and segregation cost non-uniformity in dopantconcentrations. Further, a deepened periphery 25 in the crucibleprevents the shallow melt from snapping and additionally acts as a feedcompartment.

[0033] One aspect of the invention discovery is that a crucible with adeepened periphery prevents snapping of a shallow melt.

[0034] A shallow melt is needed to reduce turbulent melt convection.Turbulence is related to the Grashof number:$G_{r} = {\frac{g\quad \beta \quad L^{3}\Delta \quad T}{v^{2}} = \frac{\text{buoyancy~~force}}{\text{viscous~~force}}}$

[0035] Where g is the gravity, β is the thermal expansion coefficient ofsilicon melt, L is the melt height, ΔT is the vertical temperaturedifference in the melt, ν is the dynamic viscosity of silicon melt.

[0036] However a shallow melt tends to ball up. The melt/gas andmelt/crucible interface have energy contents proportional to theirareas. Therefore, the melt shrinks to minimize energy. The energy perunit area can also be regarded as a surface/interface tension, or forceper unit length.

[0037] The total free energy of the melt may be defined as follows:

E=Σσ _(i) A _(i) +∫ρgdz+∫PdV(V=constant, P(z)=ρg(z _(o) −z)+P _(o)+γ(1/r₁+1/r ₂)cos θ)

[0038] where σ_(i) is the surface energies (melt/crucible interface andfree surface) per unit area, A_(i) is the area of the surfaces, ρ is themelt density, P is the internal pressure in the melt, z is the verticalcoordinate, z_(o) is vertical reference position, P_(o) is a referencepressure, γ is the surface tension of the melt, r₁ and r₂ are the twonominal radii of the melt surface curvature, and θ is the angle betweenthe tangent of the melt edge and the crucible wall on the melt side.

[0039] If the interface energy is larger than melt surface energy (i.e.,non-wetting crucible, θ>90°, the melt will tend to ball up until thesurface/interface area and radius reductions are balanced balanced bygravity potential gains, which will lead to separation of the shallowmelt from the side walls of the crucible. This causes an unstable melt,and the melt motion NM is depicted in the direction of the arrows inFIG. 3.

[0040] If both melt surface energy and interface energy are very highcompared to the gravity potential energy, the melt may retreat from oneside and travel to the other side of the crucible, causing undesiredasymmetry in the melt shape and temperature distribution. Bothsituatuations are called melt snapping. The asymmetric melt is depictedin FIG. 4.

[0041] By the use of a crucible design that has a deepened periphery,snapping of melt is prevented even though a shallow melt is still formedin the crystal growth region, as illustrated by the melt stabilized inFIG. 5. Once the melt edge reaches the baseline of the crucible center,any additional melt balling up will be difficult because the gravitypotential gain would be greater (E=Σσ_(i)A_(i)+∫ρgdz+∫PdV).

[0042] From FIG. 5, it can be seen that a round or rectangular cruciblewith a deepened periphery traps the melt around the edge, and therebyprevents snapping.

[0043] A double barrier is used to isolate heat in the feedingcompartment from that in the growth compartment, and as a result of thisconfiguration, the over heated feed compartment functions to melt solidfeeding materials and minimizes thermal impact to the growing crystal.

[0044] The heat transfer is comprised of:

[0045] Conduction∝---kΔ-T

[0046] Radiation∝---σ(ε₁T₁ ⁴−εT₂ ⁴) and

[0047] Melt flow∝-C_(p)ρ(T₁−T₂)V/L

[0048] Where k is the thermal conductivity, T is temperature, σ is theStefan-Boltzmann constant, ε is the emisivity, C_(p) is the heatcapacity of silicon melt, ρ is the silicon melt density, V is the amountof silicon melt being transferred, and L is the length of melt travel.

[0049] The double barrier (with offsetting connecting holes) crucibleserves to: Reduce ∇T;

[0050] Increase L to minimize convective transfer and to eliminate theshort path of thermally conductive melt; and

[0051] Reduce radiative heat transfer with opaque barriers.

[0052] In a primary embodiment of the invention, a round double barriercrucible is utilized for semiconductor ingot growth. A top view of around double barrier crucible is depicted in FIG. 6, and shows thegrowth compartment GC, melt connecting holes MCH, and the feedcompartment FC.

[0053] In another embodiment of the invention, a round double barriercrucible is utilized for semiconductor ingot growth. A top view of around double barrier crucible is depicted in FIG. 6, and shows thegrowth compartment GC, melt connecting holes MCH, and the feedcompartment FC.

[0054] In yet another embodiment of the invention, a rectangulardouble-barrier crucible is utilized for sheet or ribbon-shaped crystalgrowth. The rectangular double-barrier crucible is shown in FIG. 7.

[0055] Finally, the double barrier crucible of the invention utilizes aninterface thermal gradient control in that the shallow melt in thegrowth compartment operates as a thermal gradient control and allows fora much better thermal gradient control near the interface. By using aninterface heater or a heat sink under the growth compartment as shown inFIG. 2, a concave or convex interface shape may be obtained.

[0056] In a simplified one dimensional case, the crystal growth rate vis determined by the vertical thermal gradients in the crystal and inthe melt near the interface:

K _(s)(dT/dz)_(s) −K ₁(dT/dz)₁=LatentHeat*v/Area

[0057] Therefore, an increase in the crystal growth rate may be obtainedwith the combination of a heat shield employed on top of the feedingcompartment and an interface heat sink or heater under the growthcompartment. The shallow melt allows close proximity between thisinterface heat sink/heat source and the solid/liquid interface, thusaffording easier control of heat transport.

1. In a single crystal pulling apparatus for providing a Czochralski crystal grows process, the improvement of a shallow melt crucible having melt connecting holes disposed outside of a growth compartment but inside of a feed compartment to eliminate the necessity of supplying a large quantity of feed stock materials that have to be preloaded in a deep crucible to grow a single ingot, comprising: a) a gas tight container, b) a crucible with a deepened periphery to prevent snapping of a shallow melt and reduce turbulent melt convection; c) a double barrier to isolate heat in the feeding compartment from We growth compartment to melt solid feeding materials and minimized thermal impact to the growth crystal; d) an interface thermal gradient control, from which a shallow melt in the growth compartment allows for better thermal gradient control near an interface; and e) an interface heater or heat sink under the growth compartment, to provide a concave or convex interface shape;
 2. The apparatus of claim 1 wherein said double barrier comprises offsetting pass-through or passage holes to thermally isolate the feed compartment from the growth compartment.
 3. The apparatus of claim 2 wherein a heat shield is disposed on top of said feed compartment
 4. The apparatus of claim 3 wherein an interface heater is disposed under said growth compartment.
 5. The apparatus of claim 3 wherein said heat sink is disposed under said growth compartment.
 6. The apparatus of claim 2 wherein said crucible is rectangular in shape.
 7. The apparatus of claim 2 wherein said crucible is round or circular in shape.
 8. The apparatus of claim 6 wherein said heat shield is disposed on top of said heat compartment
 9. The apparatus of claim 7 wherein a heat shield is disposed on top of said heat compartment.
 10. The apparatus of claim 8 wherein an interface beater is disposed under said growth compartment.
 11. The apparatus of claim 8 where a heat sink is disposed under said growth compartment.
 12. The apparatus of claim 9 wherein an interface heater is disposed under said growth compartment.
 13. The apparatus of claim 9 wherein a heat sink is disposed under said growth compartment.
 14. The apparatus of claim 2 wherein said passage holes are near the bottom of said double barrier.
 15. The apparatus of claim 14 wherein a heat shield is disposed on top of said feed compartment.
 16. The apparatus of claim 15 wherein an interface heater is disposed under said growth compartment.
 17. The apparatus of claim 15 wherein a heat sink is disposed under said growth compartment.
 18. The apparatus of claim 14 wherein said crucible is rectangular in shape.
 19. The apparatus of claim 14 wherein said crucible is round or circular in shape. 